Isn't the argument against farmed fish more about the toxins and disease that open-pen farmed fish can spread to wild salmon, than the fact that they are farmed at all? It's been pretty widely reported that open-pen farms can be extremely destructive to native species in the farm areas.
I assume (or at least hope) most of these protesters would be fine with farmed fish from, say, a land-based fish hatchery. But those are just vastly more costly to operate than using a pen in the ocean.
That's basically the argument, yes. Does the argument align with demonstrable facts? I don't know. My understanding is there is a lot of theoretical risks associated with farming salmon in BC but we have very little definitive proof, even after years of studying the issue.
There's no doubt in my mind that the biggest threat to wild salmon is the reality that humans are eating too many of them.
Farming and more hatcheries are really the only long-term solutions for continuing to have a fishing industry in BC, in my opinion.